Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!spool.mu.edu!uunet!microsoft!gordonl
From: gordonl@microsoft.UUCP (Gordon LETWIN)
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Summary: Distributed Amps
Message-ID: <72454@microsoft.UUCP>
Date: 20 May 91 18:17:57 GMT
Article-I.D.: microsof.72454
References: <72314@microsoft.UUCP> <1991May14.164451.17@cmkrnl.uucp>
Organization: Microsoft Corp., Redmond WA
Lines: 45

In article <72314@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN)
 writes:
> I'm building a house which will have good quality ceiling speakers in 
> most rooms.  They'll play music at low volumes and act as alarm/PA/Intercom
> speakers at higher volumes.
> 
> Hooking say 20 pairs of speakers in parallel provides too low an impedance
> for the amplifier.  

Thanks again to all the people who emailed a response to this posting.
One person pointed out that the voltage that I want to follow goes from
-V to +V, not 0 to V, so this will complicate the follower circuit somewhat.
Also, since it's running class A I'll need some considerable heat sinking
capacity for when in alarm mode.

Most people mentioned 70v PA systems.  I'm aware of these systems but I'm
not interested in PA, as per above these is primarily a "hifi" music
system.  My concern is that the 70v transformers may not to too well in
the bass.  Also, I'll need a transformer for each speaker, two per stereo
pair, which will run into major bux if I get hifi ones (i.e., big), assuming
that "hifi" stuff exists in the PA market.

The most interesting idea I received is to use those cheap single-chip
audio amplifiers, one for each stereo pair.  The National data book
describes units which, for about $5, will drive about 3 watts a channel.
The chip's specs look fine; I'll verify via blind testing that I can't
hear the difference between a chip and a "high end" hifi amp through these
"nice but not amazing" wall speakers.  Three watts should be plenty loud;
the THD is about 10% at that volume, but only alarm signals would be
played that loudly.

Another advantage of this is that I can now use those single chip
DC-controlled volume/tone control chips.  I can supply the DC via
either a digital pot chip or via PWM from a microprocessor.  My house
will be heavily automated and each room will have a touch screen.
I can then write software to control the volumes via the touch screens.
Also, I can set the house into "party mode" and remotely command the
volume levels in all the rooms.  The micros are already there as part
of the house automation; the cost for the digital pot chip, the amplifer,
and the volume control comes to about $15 a pair; this is a lot less than
an amplifier, transformer, and pad, so the idea looks like a winner.

I'll post when I've tried it out...

	gordon letwin